1. Field of Use
This invention relates generally to apparatus for monitoring the terminal voltage of a storage battery and for charging and maintaining a maximum charge on the storage battery in accordance with the monitored terminal voltage.
In particular, the invention relates to an electronic charge protector employed in such apparatus to monitor terminal voltage and to control energization from an AC source of a conventional battery charger which supplies DC power to charge the battery.
2. Description of the Prior Art
Very extensive prior art exists pertaining to apparatus for charging electric storage batteries. Such apparatus typically comprise a battery charger energizable from an AC power source which converts the AC power to DC power which is supplied to the battery terminals to charge the battery. The main function of a battery charger is to cause electric current to flow back into a battery in the opposite direction from which current flowed during discharge. The current for charging can come from various sources, but presently the typical source is a stepdown transformer connected to an AC power source and a solid-state rectifier providing DC power to the battery terminals. Formerly, either motor generators or stepdown transformers with selenium rectifiers, were extensively used as chargers.
Some prior art battery chargers include protective means for sensing the level of the battery charge (i.e., battery terminal voltage) and for taking appropriate action in accordance therewith so as to prevent overcharging with its attendant risks, i.e., damage to the battery plates, damage to the charger or gasing of the plates and creation of a corrosive or explosive hydrogen-filled atmosphere. For example, some chargers automatically stop applying heavy charging power when the battery is almost or fully charged so as to prevent overcharging. Others continue to supply a steady low-power trickle or float charge to apply a full charge or to maintain the full charge after the battery is fully charged. In some cases the heavy charging power and/or trickle charge is applied as a tapered charge i.e., one in which applied voltage is gradually decreased (i.e., automatic rate reduction), either smoothly or in incremental steps. Some prior art chargers employ timers to limit charging to some arbitrarily selected time interval or to operate the charger at selected on-off time intervals; either before or after full charge is reached.
As previously explained herein, some prior art chargers employed selenium rectifiers and it was necessary to disconnect (manually or automatically) the DC charging leads from the battery terminals to prevent leakage from the battery after charging was completed. However, in battery chargers not using such rectifiers, charging can be terminated either by disconnecting the charger from its AC power source, or by disconnecting the battery from the charger, or both. Some prior art chargers included indicator devices, such as meters or lights to indicate the status of the battery being charged and the status of the charger i.e., on, off, full power, trickle power, etc. Some chargers include polarity protective means to prevent reverse polarity connection of the battery to the charger.
In the charging operation of a typical lead-acid type storage-battery, such as used in vehicles, including autos, boats, aircraft, or in stand-by power systems or the like, the following situation typically occurs.
If, for example, a battery rated at 12 volts is "down", or discharged, the battery terminal voltage will typically be under 12.5 volts, but above 11 volts. When a charger connected to this battery is turned on, the charging voltage of the charger is usually somewhere between 13.5 volts and, at a maximum, 15 volts. The moment the charger is energized, the battery builds up a counter-emf and the voltage across the battery terminals will rise much higher than the actual voltage produced in the battery i.e., the higher the charger voltage, the higher the counter-emf. Since the voltages are fairly well matched, not much current can be forced back into the battery. As a result, a battery charger, in order to charge well, has to have voltage high enough to overcome this counter-emf in order to force current into the battery at a high rate. However, when this is done, and the battery becomes charged, (assume an 80 ampere-hour rated battery and it is charged to deliver 70 of the 80 amperehours) the battery will start gassing profusely at the charge rate at the "top end" of the battery charge (called "topping off the battery"). It is very easy to charge a battery to 50% or 60%, or even 70%, of its rated ampere-hour potential, to a level below 14 volts. This is called "float charging". But, then, the battery plates don't gas as they should and a scum forms on the battery plates and this decreases the ampere-hour rating of the battery. As a result, the battery delivers a smaller potential than its rating. So-called "float charging" does solve the problem of burning up the battery due to high gassing over a period of time, but it also creates a problem in that the battery no longer has the original ampere-hour rating.